Heat shields are used for example in combustion chambers or flame tubes, which may be part of a furnace, a hot gas duct or a gas turbine and in which a hot medium is produced or ducted. Gas turbine combustion chambers which are subject to a high level of thermal loading for example are therefore lined with a heat shield to protect against excessive thermal stressing. The heat shield typically comprises a number of heat shield elements disposed on a support structure to provide cover and screening the wall of the combustion chamber from the hot combustion waste gas.
In order not to impede the thermal expansion of the heat shield elements during contact with the hot combustion waste gas, they are secured to the support structure leaving gaps between adjacent heat shield elements.
Such a heat shield on a support structure is described for example in EP 0 558 540 B1. In this heat shield rectangular ceramic heat shield elements have a hot side to face the hot waste gas, a cold side to face the support structure and four peripheral sides connecting the hot side to the cold side. The heat shield is provided in particular for attachment to the support structure of an axially symmetrical combustion chamber. The heat shield elements are retained by means of retaining elements, having a fixing section for fixing to the support structure and a retaining section to engage in grooves on peripheral sides of the heat shield elements. Those peripheral sides of the heat shield elements, in which the grooves are provided to engage with the engaging sections, extend along the axial direction of the axially symmetrical combustion chamber. Two peripheral sides provided with grooves therefore lie at opposing ends of a heat shield element when viewed in the peripheral direction of the combustion chamber.
In the heat shield in EP 0 558 540 B1 the heat shield elements are fixed in the peripheral direction of the combustion chamber by the engagement of retaining elements fixed to the support structure in the grooves of the peripheral sides. They are however not securely fixed in the axial direction of the combustion chamber, as an axial fixing system is not provided. If the tolerances are distributed unfavorably, for example if all the heat shield elements are at the lower tolerance band, the gaps between adjacent heat shield elements can increase due to displacement of the heat shield elements in the axial direction, resulting in increased penetration of hot gas into the gaps.
Generally the gaps between heat shield elements are shielded against penetration of hot gas by means of barrier air, in other words pressurized air, which flows through the gaps into the combustion chamber. If large gaps, which can occur due to axial displacement, have to be taken into account, this increases the barrier air required to block the large gaps adequately. For ceramic heat shield elements in the area of large gaps the increased flow of barrier air results in a higher temperature gradient within these heat shield elements. The increased temperature gradient in turn results in increased crack formation in the area of the edges of the ceramic heat shield elements and also in the cracks being longer than with a smaller temperature gradient.